This invention relates to electrical insulation such as primary insulation which is particularly suitable for use in high voltage cable and to a composition of polyethylene which incorporates an additive providing resistance to electrical breakdown of the cable. More specifically, this invention relates to improving the resistance to electrical failure of low density polyethylene and crosslinked polyethylene.
Electrical failure of high voltage insulation is often initiated at contaminating particles. It is extremely difficult, if not impossible, to extrude solid organic insulation, such as polyethylene, on a conductor without any flaws. Even if the polyethylene manufacturer uses extremely clean techniques, contaminants can later be introduced in subsequent handling of the resin prior to final shaping. Another cause of electrical failure in the insulation is the presence of a void.
High voltage power cables insulated with insulating polymers are subject to dielectric breakdown by a mechanism known to the trade as electrical "treeing". Treeing is a relatively slow progressive degradation of an insulation caused by electron and ion bombardment of the insulation resulting in the formation of microchannels or tubes having a tree-like appearance, hence the name. A tree initiates at points of contamination or voids which are foreign to the polymeric insulation by the action of ionization (corona) during high voltage surges. Once a tree starts it usually grows, particularly during further high voltage surges, and at some undetermined time, dielectric failure can occur.
To overcome this problem, various additives have been disclosed, particularly in polyethylene or other polyolefin, which require an increase in applied voltage to cause initiation of a tree. This use of an additive attempts to prevent failure of the insulation by preventing any formation of trees.
Maloney U.S. Pat. No. 3,499,791 discloses a coating for an electrical high voltage cable comprising a polyethylene resin which contains an inorganic ionic salt of a strong acid and a strong Zwitter-ion compound. An insulated cable provides resistance to electrical breakdown and stress cracking under the influence of corona.
Kato et al., U.S. Pat. No. 3,956,420 discloses insulation with improved electrical breakdown resistance comprising polyolefin, a ferrocene compound and a substituted quinoline compound. Also disclosed in the patent is additional use of a small amount of polyhydric alcohol, dispersant, surfactant or unsaturated polymer or mixture thereof to obtain another improvement in electrical breakdown strength.
MacKenzie, Jr. U.S. Pat. No. 3,795,646 discloses an ethylene-containing polymer composition which exhibits improved ionization resistance under high voltage stress by employing a silicone fluid in a crosslinked polyethylene composition.
Japanese Pat. No. 14348/75 relates to wire cables with improved dielectric breakdown resistance provided by an insulation of polyethylene containing 0.1% by weight of an aromatic ketone.
German Pat. No. 2147684 discloses the concept of increasing the electrical breakdown resistance of polymers, especially polyethylene, by modifying the free path of charge carriers (electrons) by incorporating additional scatter centers or by reducing the crystallinity of the polymer.
Japanese Patent Application No. 7201988 discloses insulated power cable having improved breakdown resistance by providing an insulated layer of polyethylene, polypropylene, polycarbamate or polyester containing mica particles coated with hydrophobic insulating material of silicone oil, stearic acid, palmitic acid, or oleic acid.
Japanese Kokai Pat. No. 49/119,937 discloses electric insulating resin compositions which give an increase in dielectric breakdown voltage through mixing into a resin composition such as polyethylene a ferrocene-aldehyde (or ketone) polymer having ferrocene groups or a mixture of the ferrocene containing polymer and a higher alcohol.